The present invention relates generally to digital-to-analog converters and more particularly to a system and method for a high-precision digital-to-analog converter using thermometer coding and fractional-bit DAC.
Digital-to-analog conversion is a process for converting information from a digital signal into an analog signal such as a voltage or a current. The digital signal can usually be represented as a binary number. A binary number system represents numeric values using two symbols, typically 0 and 1. Binary numbers are characterized by their having different weighting for each digit (or bit), such that each bit represents an order of magnitude greater value. For a binary number, the weighting of the bit, often referred to as the significance of the bit, doubles for each digit. For example, bit 1 is twice the value of bit 0 and bit 2 is twice the value of bit 1.
In view of the foregoing, one means for converting a binary number to an analog signal is to couple the digital signal to a logic circuit that would, in turn, control multiple current sources. Each bit of the digital signal would control a current source to provide a current proportional to the weighting of the bit. For example, the least significant bit would control one unit of current, whereas the second least significant bit would control 2 units of current. The output of each of the current sources would be coupled to a resistive network such that a voltage corresponding to the digital input signal would be generated according to the weighting of the input bit patterns. This methodology is referred to as an “R-2R” DAC because resistance R and 2R are used to construct a telescope resistance network. The R-2R DAC has several limitations when put into practice. One of these limitations is the output impedance matching for the resistive network. Another limitation is that device mismatches make it difficult to achieve high precision with monotonicity. And a third limitation to the R-2R DAC is that there may be an increased in noise as the input signal changes to a more significant bit. For example, from “0111” to “1000”. In this example, the noise is caused by turning off the current to bits 1, 2 and 3 while turning on the current to bit 4. Because of these limitations, binary coded DACs are often limited to 8 bit applications.
To overcome the limitations of the binary coded DAC described above, a “thermometer coder” may be used to convert a digital signal to an analog equivalent. Each bit of the thermometer coded signal would control a current source of equal weight, thus providing a current proportional to the fully decoded signals. Thermometer coders are used because the number of DAC cells are turned on proportional to the value of the input data. Thus, monotonicity can be ensured. This provides a lower noise analog output because there is less switching noise. The drawback to a thermometer coder is that they require a relatively large amount of area to implement on an integrated circuit. Therefore, to increase resolution, a combination of a thermometer coder combined and a conventional binary-coded DAC may be used to create a “segmented” DAC.
FIG. 1 shows a conventional segmented DAC 100. The segmented DAC 100 includes a binary input signal (b0-b9) coupled to latches 116. A first portion of the latches provides a signal that is coupled to the switches 118 and to a thermometer coder 114. The switches 118 are weighted to provide a current proportional to its respective bit (I0-I6). The thermometer coder also provides a signal to a second portion of the switches 118, which are weighted to provide substantially equal amounts of a current (I7-I13). The output of the switches 118 is coupled to a resistive network for providing a voltage (Vout) that is proportional to the binary input signal (b0-b9).
Higher level resolution may be achieved with this combined “segmented” method; however, it also requires a more sophisticated layout scheme, a relatively large integrated circuit area, and the output impedance needs to match the resistive network. Additionally, segmented DACS may also require trimming to correct for manufacturing variations and nonlinearity. For these reasons, it is desirable to have a digital-to-analog converter that uses less integrated circuit areas and provides a better quality analog output signal.